Electron microscope



April 28, 1942. s. RAMO 2,281,325

ELECTRON MI CROSCOPE Filed Aug. 20, 1941 IHAGE-REPRODUCING SURFACE QBJEbTZlEPBlfiT v 7 P 4 b.

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g4; c I I *6 Inventor: Simon Rama,

His Attorney.

Pa tentecl Apr. 28, 1942 ELECTRON MICROSCOPE Simon Ramo, Schenectady, -N. ,Y.,: assignor to General Electric Gompanmrza corporation of New York Application August v20, 1941, SerialNo. 407,562

7 Claims.

The present invention relates to electron-optical apparatus particularly adapted for the microscopic examination of electron-impervious objects.

It is known that the component rays of an electron beam may be refracted by appropriately conditioned electro-static .or magnetic fields in a manner analogous to that in which light is focused by an optical lens. This has led to the development of the so-called electron microscope as an instrument for examining bodies of such minuteness as to be outside the resolving power of the optical misc-roscope. While this instrument is finding increasing use in the investigation of objects capable of penetration by electrons, as for example, minute organic bodies, considerable difficulty has been experienced in trying to apply it to the direct examination of opaque bodies, that is, bodies of such thickness and density as to be impervious to electrons.

It is an object of the present invention to provide improved means by which electron-optical methods may be successfully employed in the study of at least the surface characteristics of bodies of any degree of impenetrability whatever.

An important feature of the invention consists in an apparatus arrangement by which electrons are caused to be grazingly reflected from a surface desired to be studied and are thereafter focused by a magnifying electron lens system on the surface on an image-reproducing agency. In the preferred embodiment of the invention the surface of the image-reproducing-means is acutely inclined with respect to the-path of the focused electrons so that the resultant image has an enlargement in one dimension which is materially greater than the magnification of the lens system as normally employed.

The features of the invention which I desire to protect herein are pointed out with particular ity in the appended claims. The invention itself, together with further objects and-advantages thereof, may best be understood by reference to-tlie following description taken in connection with the drawing in which Fig. 1 represents diagrammatically and in section an electron microscope; Fig. 2 illustrates an alternative and preferred embodiment of the invention; Fig. 3 is a schematic view explanatory of the construction of Fig. 2; and Figs. 4, 4a and 4b are representations useful in explaining the-utility of the invention.

Referring particularly to Fig. 1 there is shown an electron microscope comprising an elongated vacuum-tight container IQ of a tubular metal construction. At one end the container is closed by a glass window ll having a fluorescent material l2 on its inner surface, and atthe other end of thecontainer thereis provided a glass'insulator llxwhich serves to support an electron source in the form of a filamentary cathode IS. The cathode l5 issurrounded by a tubular metal member l6 which confines the emitted electrons to a narrow beam and is cooperatively positioned with respect to an apertured electrode l8 which is in contactwith'the metal envelope part II). In the normal use of the, apparatus the envelope I0 and the .apertured electrode I8 are maintained at ground potential and the cathode is maintained at a high negative potential (e. g. by connection to a potential source 20) so that electrons emitted from the cathode are projected axially of the container with the ultimate. object of producing ,a visible image on the fluorescent screen [2. (For some uses of the-apparatus the window I l and the screen l2 may be replaced by :a device for holding an electron-sensitive film for obtaining a photographic record.)

Between the cathode l5. and the image-reproducing screen l2 there is provided an object-supporting means indicated generally at 23. In the intended use of the apparatus this means is to be employed as a mount for an electron-impervious object desired to be examined, and in this connection the supporting structure should be provided with appropriate clamps or other means (not shown)v for holding such an object with its surface in exposedposition parallel to thetop of the part 23. The object support should also be associated with means (notshown) for permitting one objectto bereplaced by another.

.In accordance with. the present invention, the objectesupportingmeans .23 is so arranged as to hold the object selected for examination with its surface inclined at a small angle with respect to the direction of..motion A of the electron beam proceeding from the cathode l5 and inposition to intercept this beam. Under these circumstances, electrons will strike the object surface grazingly with a low-angle of incidence and will be reflected without substantial change in velocity along a path such as is-indicated by the dotted line B, the angle of reflection being approximately equal to the angle of incidence. The nature of the reflected electron stream leaving the surface of the object will naturally be a function of the condition of the object surface. For example, discontinuities or atomic patterns existing in the object surface will be reproduced as variations in the cross-sectional pattern of the electron stream. In order that these latter variations may be observed and interpreted, the electron stream is affected electron-optically in such fashion as to cause an enlarged image to be produced on the screen l2. This is accomplished by causing the stream to pass through an electron lens which,,in the, illustrated embodiment, consists .of a series ofthree apertured disks 26,11, and 28 arranged with their openduce refraction of the component rays of the electron beam and will cause an enlarged image of the object surface to be produced in the conjugate focal plane of the lens. In the present case this conjugate plane is'made to coincide with the location of the screen I2 so that the screen is caused to exhibit a visible and enlarged replica of the object.

Due to the angular inclination of the object surface with respect to the path of the impinging electrons, only a narrow strip of the object under consideration will appear in good focus on the surface of the screen H of Fig. 1. While for many purposes this result may be sufficient to yield the desired information, improved results may be obtained by an arrangement such as that shown in Fig. 2. In this case most of the structural elements of the illustrated microscope are identical with elements shown in Fig. 1 and therefore bear corresponding numbers. However, the image-reproducing means, shown as a fluorescent screen 35 adapted to be viewed through a window 38, is inclined at a sharp angle with respect to the path of the central ray of the electron stream proceeding from the objectsupport 23. y

The preferred relationship of parts for this construction is that shown in Fig. 3, in which the angle of incidence of electrons impinging on the object-support is represented as The angle between the axis of the electron lens and the object surface is also made equal to since this coincides with the angle of reflection of most of the reflected electrons. Under these circumstances, the magnification of the resultant image in a direction parallel to the path of the electron stream is M as compared with a magnification M in the other dimension. This result may be explained by the following analysis in which the character I represents the focal length of the lens system in question, p represents the object distance, and q represents the image distance.

If it be assumed that p is a variable quantity (as is true for various points on the object surface in the arrangement of Fig. 2) then q must also vary for these points. In order to determine the nature of the variation of q and p we may note the following relationship as being characteristic of lens systems solvingfor q, we have Pf q and fl dp 2 f (pf) (3) However, p is very'nearly equal to f, and hence p-f is a very small quantity in the type of system under consideration. Thus Pf is very large compared to unity, and the second term of Equation 3 (being of the order of the square of the first term) is consequently much larger than the first term. The first term is in fact negligible in relation to the second. We may therefore write:

a? (p z( p Referring again to Equation 1 we note that 5 f p+q Accordingly J 6 p f PM PM Since p is very small in relation to q, it may be dropped from the denominator of (6) without materially affecting the value of the quotient, and we have 2 P f q pp (7) substituting in (4) we find d fq q i T (Since p=f, approximately). Taking g g p as a definition of longitudinal magnification (as it is in fact), and noting that the normal transverse magnification of a lens system is given by withrespect to the central path of the focused electrons. This is the situation represented in Fig. 3.

It should be pointed out that this aspect of the invention, that is to say, the use of an inclined image-reproducing surface to take advantage of longitudinal magnification effects, can also be usefully employed in the examination of a selected planar section of a transparent object. This is done by arranging the section in question at an angle with respect to the path of electrons projected through the object and receiving the electrons (after submission to appropriate lens action) on a reproducing surface having an inclination with respect to the electron path. 7

Assuming M to be a large figure, such as is readily obtainable with an electron lens of appropriate construction, the value M becomes relatively enormous, so that its realization in an arrangement of the type shown in Fig. 2, even though limited to one dimension, permits the attainment' of information not otherwise possible to obtain.- 'I his point may be further clarified by a consideration of Figs. 4, 4a, and 4b, in which 4 represents (in enlarged form) a microscopically small object o desired to be examined; 4a shows the magnification of such object by means of a symmetrically acting lens having a normal magnification of three; and 4b illustrates the magnification of the same object as obtained by a similar lens system employed in an arrangement like that of Fig. 2. It will be seen that while the image of Fig. 4b is a distorted one, it affords information with respect to the mode of connection of the various elementary parts of the object under investigation which is not provided by the symmetrical image of Fig. 4a.

In either of the arrangements of Figs. 1 and 2, it is desirable that the angle of incidence of the electron stream on the object under examination be as small as possible, preferably less than five degrees. The angle which can be successfully employed in this connection will be determined mainly by the condition (e. g. smoothness) of the object surface, and for very smooth surfaces it can readily be made less than one degree.

Where the angle (Fig. 3) is very small, it is apparent that the screen angle will be very minute indeed, so that some difi'iculty might be anticipated in establishing such an angle with any degree of exactness. This difficulty is mitigated, however, by the fact that a considerable departure from the ideal value of this angle may be tolerated Without producing any great imperfection in the resultant image. This follows from the fact that if the angle is very small, a minor change in it will produce only a negligible change in the axial position of the various parts of the image-reproducing surface.

While the invention has been described by reference to constructions which employ an electrostatic lens system, it may also be used in connection with magnetic lenses. I aim to cover in the appended claims this and all other equivalent variations which come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. Apparatus for the microscopic examination of an electron-impervious object comprising means for generating a beam of electrons, means for supporting the chosen object with a surface desired to be examined in the path of the electron beam and inclined thereto to assure a low angle of incidence of impinging electrons, an electron lens system for electron-optically focusing the electrons grazingly reflected from the said surface, and electron sensitive image-reproducing means positioned to receive the focused electrons.

2. Apparatus for the microscopic examination of an electron impervious object, said apparatus comprising means for projecting a beam of electrons against a surface of the object with an angle of incidence less than about five degrees, a convergent electron lens system for focusing electrons grazingly reflected from the said surface, and electron sensitive image-reproducing means positioned to receive the focused electrons.

3. Apparatus for the microscopic examination of an electron-impervious object comprising means for projecting a beam of electrons against a selected surface of the object with a low angle of incidence, an electron lens system for focusing electrons grazingly reflected from the said surface, said lens system having its axis of symmetry inclined to the said object surface at an angle equal to the said angle of incidence of the projected electron beam, and image-reproducing means positioned to receive electrons focused by the said lens system.

4. Apparatus for the microscopic examination of an electron-impervious object comprising means for projecting a beam of electrons against a selected surface of the object with a low angle of incidence, an electron lens system for focusing electrons grazingly reflected from the said surface and an image-reproducing surface positioned to receive electrons focused by the said lens system, the last-named surface being angularly inclined with respect to the path of the central ray of the electron beam so that the resultant image has greater magnification in one dimension than in the other.

5. Apparatus for the microscopic examination of surfaces, comprising means for projecting a beam of electrons against a selected surface with a low angle of incidence c, an electron lens of magnifying power M positioned to focus electrons grazingly reflected from the said surface, and an image-reproducing surface for receiving electrons focused by said lens, said last-named surface being inclined with respect to the central ray of the focused electron beam by an angle whereby the magnification of the resultant image in one dimension is M.

6. An electron microscope comprising means for producing a directed beam of electrons, means for supporting an object desired to be examined in the path of said beam, an electron lens positioned to act on the beam after it leaves the said object, and image-reproducing means arranged approximately at the image plane of the said lens and in position to intercept the beam after its traversal of the lens, said image-reproducing means being angularly inclined with respect to the path of the central ray of the said beam whereby the magnification of the resultant image in one direction materially exceeds that in the other direction.

7. An electron optical device for microscopically examining a selected planar section of an object desired to be studied comprising means for directing a beam of electrons upon the object along a path which is inclined at an angle c with respect to the said planar section, an electron lens of normal magnification M arranged to act on the beam as it proceeds from the object, and image-reproducing means located approximately at the image plane of the said lens and in position to intercept the said beam after its traversal of the lens, said means having its surface inclined at an angle with respect to the path of the central ray of the said beam, whereby the magnification of one dimension of the resultant image is equal to M SIMON RAMO. 

